One of the most significant challenges involved in the packaging of semiconductor chips is to avoid the warping of, and other damage to, the substrate during the packaging process.
As various industry sectors are seeking thinner and lighter semiconductor chips, the semiconductor manufacturing industry is moving towards the use of semiconductor packages formed of one or more integrated circuit dies, mounted on a thin core or coreless substrate.
However, as the thickness and/or density of the core are reduced, or as separate cores are eliminated entirely, the packaging and subsequent processing of the substrates becomes more difficult. This is principally because of the decreased strength and rigidity of the substrate and the resulting vulnerability during assembly and post assembly processes and handling. One problem is the warping of the substrate, particularly with thin core and coreless substrate packages, that can result during the manufacturing processes when, for example, dies and other components are attached to the substrate.
The various manufacturing steps may involve significant increases in the temperature of the substrate, which can cause warping. The warping of the substrate can cause die cracking and result in an inoperative semiconductor chip. Another problem related to substrate warping may be poor substrate co-planarity that may render the semiconductor chip package more difficult to interconnect with other devices.
The general use of stiffener rings to support or add rigidity to a substrate is known. Also, the use of heat spreaders that are secured to the substrate, to disperse heat during operation of the integrated circuit package, is also known. These components are typically mounted to the substrate by adhesive or solder. However, difficulties can arise in the mounting of these components. During mounting of the stiffener or heat spreader components, warping or other damage can occur to the substrate.
One particular process that may involve significantly increasing the temperature of the substrate and other elements of the package, and which may render the substrate particularly prone to warping, is the under-filling of the flip chip dies. Under-filling is used to compensate for differences in thermal expansion rates between the die and the substrate and to absorb physical shock stresses incurred during use of the package. The under-fill material may be an engineered epoxy. The under-fill may be flowed into the desired positions in the gaps between the die and the substrate. Once the under-fill is cured, the chip, under-fill, and substrate deform together as a unit to significantly limit the relative deformation between the chip and the substrate.
However, when applying the under-fill, which is typically an epoxy, the heat associated with this step may result in thermally generated stresses in the substrate that may tend to warp the substrate. This may result at least in part from the fact that the die may have a coefficient of thermal expansion (CTE) that is significantly lower than the CTE of the substrate. For example the CTE of the die may be about 5 ppm and the CTE of the substrate may be about 16 ppm. The substrate and die may expand during the die attachment process and during the under-fill process when the binding epoxy is applied to bind the die and substrate together. Both the die attach process and the under-fill process, are normally carried out with the die and substrate at elevated temperatures. Once the combination of substrate and die are cooled with the epoxy binding the substrate and die together, warping may result due to the different CTE's of the substrate and die.
In known methods of making semiconductor chip packages, particularly when the packages are made from thin core or core-less substrates are being made, the attachment of the stiffener ring is normally carried out after the under-fill process. This is usually to avoid stressing the die bumps and the connection between the die and the substrate, during the attachment of the stiffener ring (ie. the under-fill reduces the risk of any relative movement of the die and the substrate at the solder bump/pad connection when the stiffener ring is attached.) However, the result is that during the under-fill process, there may not be sufficient support for the substrate, to resist warping.
Typically, in the manufacturing of semiconductor chip package, a substrate panel is utilized which can form the substrate for multiple separate semiconductor chips. Components for multiple chips may be added at the same time to a larger substrate panel. After component mounting, the larger substrate panel and components combination can be singulated (i.e. separated) into multiple individual semiconductor chip packages.
However, mounting components for multiple semiconductor chips to a single substrate panel will increase the aforementioned problems of handling and processing. Additionally, the overall size of the substrate panel can increase the stresses, at least in certain parts of the panel, during handling and processing.
Accordingly, an improved packaging process for semiconductor chips, particularly those having thin core or coreless substrates, is desirable.